The present invention relates to the field of document scanning. More specifically, the present invention addresses the problems of scanning an image of a page into a photocopier or scanner when the page cannot be laid flat on a scanning bed.
Optical scanning of an image on paper for the purposes of photocopying or generation of digital image data is well known in the art. Digital image data is often represented as an array of pixels, known as a raster-scan representation. Each pixel has a position and a value, so a pixel can describe where and what color an individual dot should be when the image is printed or displayed. While the paper image need not originate on paper, since images can be on any surface, the term is used to distinguish between it and a digital image, which exists as data in a computer memory. For some applications, optics which can be moved over a paper image, by rolling or otherwise, are acceptable. However, for higher speed and higher accuracy scanning, flat-bed optics are required.
With flat-bed optics, the paper image is held onto a flat glass surface, or platen, and an electronic image in the form of analog or digital data is generated by illuminating the paper image and sensing the reflected light. Two common methods for sensing the reflected light are taking a snapshot of the image, sensing all light from the paper image at once, and scanning the paper image, sensing the light from the paper image a point or line at a time. Regardless of how the paper image is sensed, the sensing optics are focused at the plane where the paper meets the platen, so that a paper image placed against the platen is in focus with respect to the optical sensors.
Often, it is not possible to place the entire paper image against the platen, such as when the paper image is bound into a book. In such cases, the inner margin of the paper image, where the page meets the binding, will not be flush with the platen, but the paper image will gradually separate from the platen as the page nears the binding. Where the paper image separates from the platen, points on the sensed image are non-linearly distorted from the original image. This warping, or splay, is most evident where the paper image consists of straight lines of text. Due to the optics used, lines of text will appear to rise near the bottom of the page and drop near the top of the page. Also, since a page curving away from the platen presents a smaller projection to the optics than a page flat on the platen, the text also appears compressed.
Photocopiers are available where the platen is located near the edge of the photocopier, thereby allowing a book page to be placed flat on the platen while the facing pages of the book drape down the side of the photocopier. While such photocopiers have limited applications, they suffer from several drawbacks. One is that facing pages cannot be simultaneously sensed. Other problems include paper transport difficulties and copier packaging. Also, with such copiers, two facing pages cannot be copied without a user moving the book.
This problem has been partially addressed by height measuring hardware combined with adjustable optics. Such a system is shown in U.S. Pat. No. 4,980,720. In the system disclosed therein, a height sensor is used to measure the distance (in the "z" direction) from a book page to a copy machine platen as a function of the distance from the book's spline (the "x" direction). This height function is then used to adjust mirrors, lens, and light sources so that the image reflects a constant illumination, stays in focus, and is scanned at a constant speed relative to the page.
While such a system may correct for the splay of a page being copied, such a system cannot correct for the splay of the page once an image of the page has been taken. Instead, the correction must be performed as the image is captured. Since the system assumes height is not a function of the "y" direction (i.e., height is constant along a line parallel to the spline from the top to the bottom of the book) the system cannot properly correct for the splay of a page in a book when the top and bottom of the book are not at the same height. This is often the case when the user closes a copy machine cover onto the book, pressing down more heavily on the end of the book closest to the hinge of the copy machine cover.
From the above it is seen that an improved method and apparatus for correcting for splay is needed.